1. Field of the Invention
The present invention relates to thermoformable polyolefin sheets. More particularly, the present invention relates to a thermoformable polyolefin laminate that provides improved paintability and scratch resistance in addition to the inherent and desirable properties of polyolefin.
2. Description of the Prior Art
Thermoforming is a process for converting thermoplastic sheets into useful shaped articles, which is usually accomplished by heating a thermoplastic sheet above its softening point and forcing it against a mold by vacuum, air, or mechanical pressure to a shape corresponding to the mold contour. Thermoforming of high impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) copolymer sheets have been utilized extensively in various industries. For example, thermoforming refrigerator-door liners from HIPS and ABS sheets are well established practices in the home-appliance industry. Examples of other thermoformed articles made from these polymers include food-container liners, containers for dairy products, luggage and the like. However, HIPS polymers are susceptible to chemical attack by common household chemicals, and ABS copolymers are relatively expensive and are not solvent resistant.
Recent development in polyolefins have made thermoforming from prefabricated polyolefin sheets on conventional thermoforming equipments practical, and, consequently, the use of such thermoformable polyolefin sheets has increased substantially. Polyolefins are relatively inexpensive polymers that offer desirable physical properties, including good impact resistance over a wide temperature range, and good resistance to various chemicals and moisture, as well as excellent processability. However, polyolefins do not provide hard surfaces that are scratch resistant. Furthermore, polyolefins, unlike other costly engineering polymers such as polyamides and polycarbonates, do not provide aesthetically pleasing glossy surfaces. Yet another disadvantageous aesthetic property of polyolefins is that articles formed therefrom are difficult to paint since most commercially available paint formulations do not securely and durably adhere to polyolefins.
There have been many attempts in the prior art to improve undesirable properties of polyolefins. One of such attempts is disclosed in U.S. Pat. No. 4,914,138 to Percec et al. Percec et al. teaches a blend composition comprising a functionalized polyolefin and a nitrile copolymer. However, the blend composition may not be suitable for certain applications since, as is well known in the art, a blend composition not only exhibits desirable properties of the component polymers but also retains undesirable properties. Furthermore, the desirable properties of each polymers-in the blend tend to be proportionally attenuated by the amount of other polymers present in the blend. Although nitrile copolymers are known for their high gas barrier properties, optical clarity and good chemical resistance, nitrile copolymers do not have desirable impact resistance, moisture barrier properties. In addition, polyolefins and nitrile copolymers are not compatible and cannot be blended to retain any useful properties unless functionalized polyolefins are used. As such, the extra processes and costs required in functionalizing the polyolefins and the subsequent blending procedures render the blend more expensive.
It is, therefore, desirable to have modified thermoformable polyolefin sheets that combine desirable properties of polyolefins and nitrile copolymers and, yet, are not substantially more expensive than unmodified thermoformable polyolefin sheets.